Terahertz technology is a fast growing field with variety of applications in biology and medicine, medical imaging, material spectroscopy and sensing, monitoring and spectroscopy in pharmaceutical industry, security, and high-data-rate short-range communications. Among different terahertz sources, photomixers are potentially compact, low power consuming, coherent, low-cost, and tunable continuous-wave sources. A terahertz photomixer is a heterodyne scheme, in which two laser beams with their frequency difference falling in the terahertz range mix in a nonlinear medium, such as a photoconductor or a superconductor, and generate a signal, whose frequency is equal to the frequency difference of the two lasers. The frequency of the generated terahertz signal can be tuned by tuning the central frequency of one of the lasers. <br ><br /> In this dissertation, the photomixing in superconductors and photoconductors is studied, and comprehensive analytical models for the interaction of two interfering laser beams with these materials are developed. Integrated photomixer-antenna elements as efficient terahertz sources are introduced and arrays of these elements as high power terahertz sources are designed. Also, an array of photoconductive photomixer-antenna elements with integrated excitation scheme is proposed. <br ><br /> In a photo-excited superconductor, the fundamental equations for the motion of the carriers inside the superconductor material are used in connection with the two-temperature model to find an analytic expression for the generated terahertz photocurrent inside the film. In a photo-excited photoconductor, the continuity equations for the electron and hole densities are solved in their general form along with the appropriate boundary conditions to find photocurrent distribution inside the photoconductor film. It is shown that in a continuous-wave (CW) terahertz photomixing scheme, the resulting photocurrent contains a dc component and a terahertz traveling-wave component. The dependency of the amplitude and the phase of the generated photocurrent on the physical parameters of the photomixer, the parameters of the lasers, the applied dc bias, and the configuration of the device is explored in detail for a photoconductive photomixer made of low-temperature grown (LTG) GaAs and for a high-temperature superconductive photomixer made of YBa₂Cu₃O_7-&delta;. <br ><br /> The developed models for the photoconductive and the superconductive terahertz photomixers are used to design new integrated photomixer-antenna devices. In these devices, the photomixing film simultaneously acts as an efficient radiator at the terahertz frequencies. Integrating the photomixing medium with the antenna not only eliminates any source to antenna coupling problem, but also makes the proposed device attractive for array configurations. <br ><br /> To increase the generated terahertz power, arrays of the photoconductive and the superconductive photomixer-antenna elements are proposed as CW terahertz sources. It is shown that a sub-milliwatt terahertz power is achievable from a typical superconductive photomixer-antenna array structure. The beam steering capability of the proposed devices is also investigated. <br ><br /> A photoconductive photomixer-antenna array with integrated excitation scheme is proposed, in which the laser beams are guided inside the substrate and excite the photomixer elements. In this way the laser power is only being consumed by the photomixer elements, and the photomixer-antenna elements can be integrated with other optical components on a single substrate. The whole structure is robust and less sensitive to vibration and other environmental parameters.

Terahertz technology is a fast growing field with variety of applications in biology and medicine, medical imaging, material spectroscopy and sensing, monitoring and spectroscopy in pharmaceutical industry, security, and high-data-rate short-range communications. Among different terahertz sources, photomixers are potentially compact, low power consuming, coherent, low-cost, and tunable continuous-wave sources. A terahertz photomixer is a heterodyne scheme, in which two laser beams with their frequency difference falling in the terahertz range mix in a nonlinear medium, such as a photoconductor or a superconductor, and generate a signal, whose frequency is equal to the frequency difference of the two lasers. The frequency of the generated terahertz signal can be tuned by tuning the central frequency of one of the lasers. <br ><br /> In this dissertation, the photomixing in superconductors and photoconductors is studied, and comprehensive analytical models for the interaction of two interfering laser beams with these materials are developed. Integrated photomixer-antenna elements as efficient terahertz sources are introduced and arrays of these elements as high power terahertz sources are designed. Also, an array of photoconductive photomixer-antenna elements with integrated excitation scheme is proposed. <br ><br /> In a photo-excited superconductor, the fundamental equations for the motion of the carriers inside the superconductor material are used in connection with the two-temperature model to find an analytic expression for the generated terahertz photocurrent inside the film. In a photo-excited photoconductor, the continuity equations for the electron and hole densities are solved in their general form along with the appropriate boundary conditions to find photocurrent distribution inside the photoconductor film. It is shown that in a continuous-wave (CW) terahertz photomixing scheme, the resulting photocurrent contains a dc component and a terahertz traveling-wave component. The dependency of the amplitude and the phase of the generated photocurrent on the physical parameters of the photomixer, the parameters of the lasers, the applied dc bias, and the configuration of the device is explored in detail for a photoconductive photomixer made of low-temperature grown (LTG) GaAs and for a high-temperature superconductive photomixer made of YBa₂Cu₃O_7-&delta;. <br ><br /> The developed models for the photoconductive and the superconductive terahertz photomixers are used to design new integrated photomixer-antenna devices. In these devices, the photomixing film simultaneously acts as an efficient radiator at the terahertz frequencies. Integrating the photomixing medium with the antenna not only eliminates any source to antenna coupling problem, but also makes the proposed device attractive for array configurations. <br ><br /> To increase the generated terahertz power, arrays of the photoconductive and the superconductive photomixer-antenna elements are proposed as CW terahertz sources. It is shown that a sub-milliwatt terahertz power is achievable from a typical superconductive photomixer-antenna array structure. The beam steering capability of the proposed devices is also investigated. <br ><br /> A photoconductive photomixer-antenna array with integrated excitation scheme is proposed, in which the laser beams are guided inside the substrate and excite the photomixer elements. In this way the laser power is only being consumed by the photomixer elements, and the photomixer-antenna elements can be integrated with other optical components on a single substrate. The whole structure is robust and less sensitive to vibration and other environmental parameters.

The frequency demands of radiating systems are moving into the terahertz band with potential applications that include sensing, imaging, and extremely broadband communication. One commonly used method for generating and detecting terahertz waves is to excite a voltage-biased photoconductive antenna with an extremely short laser pulse. The pulsed laser generates charge carriers in a photoconductive substrate which are swept onto the metallic antenna traces to produce an electric current that radiates or detects a terahertz band signal. Therefore, analysis of a photoconductive antenna requires simultaneous solutions of both semiconductor physics equations (including drift-diffusion and continuity relations) and Maxwell’s equations. A multi-physics analysis scheme based on the Discontinuous-Galerkin Finite-Element Time-Domain (DGFETD) is presented that couples the semiconductor drift-diffusion equations with the electromagnetic Maxwell’s equations. A simple port model is discussed that efficiently couples the two equation sets. Various photoconductive antennas were fabricated using TiAu metallization on a GaAs substrate and the fabrication process is detailed. Computed emission intensities are compared with measured data. Optimized antenna designs based on the analysis are presented for a variety of antenna configurations.

This thesis is concerned with terahertz (THz) technology from both optical and electronic approaches. On the optical front, the investigation of optimised photoconductive switches included the characterisation, fabrication and testing of devices which can generate and detect THz radiation over the frequency range from DC to ~ 2.5 THz. These devices incorporated semiconductor photoconductors grown under low temperature (LT) Molecular Beam Epitaxy (MBE) conditions and using distributed Bragg reflectors (DBRs). The material properties were studied via numerous characterisation techniques which included Hall Effect and mid infrared reflections. Antenna structures were fabricated on the surface of the active layers and pulsed/continuous wave (CW) signal absorbed by these structures (under bias) generates photocurrent. With the help of the DBRs at certain wavelengths (800 nm and 1550 nm), the absorption coefficient at the corresponding illumination wavelength increased thus leading to significant increase of the THz output power while the materials kept the desirable photoconductive material properties such as high dark resistivity and high electron mobility. The inclusion of DBRs resulted in more than doubling of the THz peak signals across the entire operating frequency range and significant improvements in the relative THz power. For the THz electronic approach, a new type of InP-based Asymmetric Spacer Tunnel Diode (ASPAT), which can be used for high frequency detector, was studied. The asymmetric DC characteristics for this novel tunnel diode showed direct compatibility with high frequency zero-bias detector applications. The devices also showed an extreme thermal stability (less than 7.8% current change from 77 K to 400 K) as the main carrier transport mechanism of the ASPAT was tunnelling. Physical models for this ASPAT diode were developed for both DC (direct current) and AC (alternating current) simulations using the TCAD software tool SILVACO. The simulated DC results showed almost perfect matches with measurements across the entire temperature range from 77 K to 400 K. From RF (radio frequency) measurements, the intrinsic diode parameters were extracted and compared with measured data. The simulated zero biased detector circuits operating at 100 GHz and 240 GHz using the new InGaAs-AlAs ASPAT diode (4*4 micrometer square) showed comparable voltage sensitivities to state of the art Schottky barrier diodes (SBDs) detectors but with the added advantage of excellent thermal stability.

Le sujet de la thèse a porté sur la mise au point, la caractérisation et l'utilisation de matériaux semi-conducteurs, au sein desquels les porteurs libres ont un temps de vie extrêmement brefs (picoseconde ou sub-picoseconde), pour réaliser des antennes photoconductrices émettrices ou détectrices de rayonnement électromagnétique térahertz (THz). Contrairement au semi-conducteur LTG-GaAs (low temperature grown GaAs) à la technologie bien dominée et aux performances exceptionnelles lorsque photo-excité par des impulsions lasers de longueurs d'onde typiquement inférieures à 0,8 µm, le travail portait ici sur des matériaux permettant l'emploi de lasers dont les longueurs d'onde sont celles des télécommunications optiques, à savoir aux alentours de 1,5 µm. L'intérêt est de bénéficier de la technologie mature de ces lasers, et du coût relativement modique des composants pour les télécommunications optiques. Pour réaliser des antennes THz performantes et efficaces, le matériau semi-conducteur doit présenter plusieurs qualités : vie des porteurs libres très courte, grande mobilité des porteurs, haute résistivité hors éclairement, et bonne structure cristallographique pour éviter les claquages électriques. Pour obtenir une courte durée de vie, on introduit un grand nombre de pièges dans le semi-conducteur, qui capturent efficacement les électrons libres. Pour les matériaux de type InGaAs employés à 1,5 µm, le problème est que le niveau en énergie de ces pièges, par exemple pour les matériaux épitaxiés à basse température, est très proche de la bande de conduction du semi-conducteur. Cela est équivalent à un dopage n du matériau, ce qui en diminue fortement sa résistivité hors éclairement. Plusieurs solutions ont été apportées par différents laboratoires : compensation par dopage p pour les matériaux épitaxiés à basse température, bombardement ionique, implantation ionique, ou même structures à couches alternées où la photo-génération et la recombinaison des porteurs libres se produisent à des endroits différents. Le but du travail de thèse était de fabriquer des matériaux préparés suivant ces différentes techniques, de les caractériser et de comparer leurs performances pour l'optoélectronique THz. Les semi-conducteurs à étudier étaient de type InGaAs comme déjà publiés par la concurrence, l'originalité de thèse portant sur la comparaison de ces différents matériaux et si possible leur optimisation,. Au cours de ce travail de thèse, de nombreuses couches d'InGaAs ont été épitaxiées, en faisant varier les paramètres de dépôt, et des antennes THz ont été fabriquées. Les couches ont été caractérisées du point de vue cristallographique, ainsi que pour la conductivité électrique DC (mesures 4 pointes, mobilité Hall…), les propriétés d'absorption optique (spectroscopie visible et IR), la durée de vie des porteurs par mesure optique pompe-sonde. Pour les couches épitaxiées à basse température, l'influence d'un recuit thermique ainsi que du dopage en béryllium ont été étudiés. Dans le cas de couches bombardées ou implantées, plusieurs ions ont été utilisés, le brome, le fer et l'hydrogène. Les relations entre la cartographie des défauts structuraux et/ou des ions implantés et les propriétés électriques et de dynamique des porteurs ont été examinées en détail. Ces études permettent de comprendre le type de défauts qui piègent les porteurs dans ces matériaux, ainsi que leur formation lors du processus de fabrication et de traitement des couches. Finalement les meilleures couches fabriquées présentent des performances comparables à celles publiées par ailleurs. Les derniers travaux de thèse ont permis d'obtenir les premiers signaux de rayonnement THz générés par une antenne fabriquée avec l'InGaAs optimisé. / The subject of the thesis focused on the development, characterization and use of semiconductor materials, in which the free carriers have a very short lifetime (picosecond or sub-picosecond) to produce photoconductive antennas emitting and detecting electromagnetic terahertz (THz) radiation. Unlike semiconductor LTG-GaAs (low temperature grown GaAs) which is a well-dominated technology and present exceptional performances when photoexcited by typically less than 0.8 micron wavelength laser pulses, the work focused on here materials for the use of lasers whose wavelengths are those of the optical communication, namely around 1.5 microns. The interest is to benefit from the mature technology of these lasers, and relatively low cost components for optical telecommunications. To achieve effective and efficient THz antennas, the semiconductor material must have several qualities : lifetime of free carriers very short, high carrier mobility, high resistivity outside lighting, and good crystallographic structure to prevent electrical breakdown. For a short lifetime, a large number of traps are introduced into the semiconductor, which effectively capture the free electrons. For InGaAs materials used at 1.5 microns, the problem is that the energy level of the traps, for example, the epitaxial material at low temperature is very close to the conduction band of the semiconductor. This is equivalent to an n-doped material, what greatly reduces its resistivity outside illumination. Several solutions have been made by different laboratories : compensation for the p-doped epitaxial materials at low temperature, ion bombardment, ion implantation, or even alternating layer structures where photo-generation and recombination of free carriers occur in different places. The aim of the thesis was to produce materials prepared using these techniques to characterize and compare their performance to THz optoelectronics. The studied InGaAs-based semiconductors were as previously published by the competition, the originality of the thesis was on the comparison of these different materials and if possible their optimization. During this work, many of InGaAs layers were grown epitaxially by varying the deposition parameters, and THz antennas were fabricated. The layers were characterized from the crystallographic point of view, as well as the DC electrical conductivity (measures 4 points, Hall mobility ... ), the optical absorption properties (visible and IR spectroscopy ), the lifetime of carriers by optical pump-probe measurement. For low temperature epitaxial layers, the influences of thermal and doping beryllium annealing were studied. In the case of shelled or implanted layers, several ions were used, bromine, iron and hydrogen. The relationship between the mapping of structural defects of the implanted ions and electrical and carrier dynamics properties were discussed in detail. These studies allow us to understand the type of defects that trap carriers in these materials, as well as training in the process of manufacturing and processing layers. Finally the best layers are made comparable to those published elsewhere performance. The last study allowed to achieve the first signals of THz radiation generated by InGaAs-based optimized antenna.

Terahertz time-domain spectroscopy systems were developed and used for the anaylsis and characterization of various materials. By using ultra-fast Ti:Sapphire and Er-doped fiber lasers, terahertz time-domain spectrometers of different configurations were constructed and tested. To increase the accuracy and sensitivity of the measurements, the systems were optimized for spectroscopic analysis. MBE grown nominally undoped epitaxial GaAs samples were used for the spectroscopic measurements. These samples were first charactrized by electrical measurements in order to check the accuracy of the terahertz time-domain experiments. We have shown that the terahertz time-domin spectroscopic techniques provides a quick way of the determining the real ( ) and complex () components of the refractive index of material. In addition, we have investigated the photoexcitation dynamics of these GaAs samples. We have demonstrated that direct and photoexcited terahertz time-domain measurements give an estimate of the carrier densities and both the hole and electron mobility values with good precision. rnin An algorithm is developed to prevent the unwanted Fabry-Perot reflections which is commonly encountered in Terahertz Spectroscopy systems. We have performed terahertz time-domain transmission measurements on ZnTe &lt / 110&gt / crystals of various thicknesses to test the applicability of this algorithm. We have shown that the algorithm developed provides a quick way of eliminating the &ldquo / etalon&rdquo / reflections from the data. In addition, it is also shown that these &ldquo / etalon&rdquo / effects can be used for the frequency calibration of terahertz time-domain spectrometers.

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